1. Field of the Invention
The present invention relates to an image display device for displaying images, such as television images.
2. Description of Related Art
In recent years, with the reduction in thickness of image display devices for displaying images such as television images, image display devices using a flat display panel such as a plasma display panel and a liquid crystal panel have become a mainstream, instead of conventional CRTs.
FIG. 4 is a perspective view showing a plasma display device as an example of such a flat image display device, viewed from the image display screen side. As indicated in FIG. 4, a plasma display device 100 has a configuration in which a plasma display panel 2 (hereinafter referred to as “PDP”), circuit boards for driving the PDP 2 and the like are covered by a front cover 3 and a back cover 4.
It should be noted that the upward and downward direction means the upward and downward direction of a plasma display device in the normal use position (that is, the vertical direction), and the left and right direction means a direction orthogonal to this (that is, the horizontal direction) in the following description.
FIG. 5 is a schematic sectional view showing the internal configuration of the conventional plasma display device 100. As indicated in FIG. 5, the PDP 2 for displaying images, a chassis 5 supporting the PDP 2 on the front surface, and a plurality of circuit boards 6 that are mounted to the back surface of the chassis 5 for driving the PDP 2 are accommodated inside the space surrounded by the front cover 3 and the back cover 4 in the plasma display device 100.
Here, the PDP 2 is likely to have a high temperature because of the principle of operation by which images are displayed using gas discharge. When the PDP 2 has a high temperature, the electrical capacity of the electrodes formed inside the PDP 2 changes, thus causing adverse effects such as the failure of normal discharge. For this reason, in order to maintain the PDP 2 at a predetermined temperature (for example, 70 to 80° C.) or less, a resin sheet 7 with high heat conductivity is interposed between the PDP 2 and the chassis 5, as indicated in FIG. 5. In this way, the heat in the PDP 2 is conducted effectively to the chassis 5, which allows the heat radiation effect and the temperature equalization effect to be obtained simultaneously.
Further, an air inlet 8 is provided in the lower part of the back cover 4, an air outlet 9 is provided in the upper part thereof, and a fan 10 is provided inside the back cover 4 so as to cover the air outlet 9. When the fan 10 is operated, airflow occurs between a space 11 inside the back cover 4 and the outside thereof, as illustrated by the arrows a and b in the figure. Thus, air at low temperature is taken in through the air inlet 8, so that the chassis 5 and the circuit boards 6 inside the back cover 4 are cooled by forced convection. Thereafter, air at high temperature is exhausted through the air outlet 9. It should be noted that although the air inlet 8 and the air outlet 9 each are a group of fine through holes in practice, FIG. 5 simply illustrates their existing locations generally.
Meanwhile, there is further progress recently in reducing the thickness of image display devices (for example, about 25 mm), and the depth of the space 11 defined by the chassis 5 and the back cover 4 has been decreasing significantly. With this reduction, the gap between the chassis 5 and the air-inlet surface of the fan 10 that faces the chassis 5 also is narrowed, which increases the resistance in the flow path, thereby reducing the amount of air to be moved by the fan 10. This deteriorates the cooling efficiency when cooling the chassis 5 and the circuit boards 6 inside the back cover 4 using forced convection. In order to prevent this, it is conceivable to increase the amount of the air to be moved by the fan 10 by increasing the driving voltage applied to the fan 10 to achieve a higher rotation rate of the fan 10. However, in that case, the noise from the fan 10 also increases, which interferes with the appreciation of the image display device. Therefore, it is desirable to reduce the noise from the fan 10.
For example, JP 2-19999 B proposes a structure for reducing the noise of a fan, relating to a computer housing, though it has no relationship to an image display device. In this structure, an air-outlet duct is provided so as to be connected to a fan discharge for cooling the inside of a housing, and a sound absorbing material in thin-layer form is attached onto the inner wall of the air-outlet duct.
FIG. 6 indicates a configuration in which the fan-noise reduction structure proposed in JP 2-19999 B is applied to the above-mentioned plasma display device 100 (when being wall mounted). That is, an air-outlet duct 18 is provided so as to be connected to the air outlet 9 of the back cover 4, and a sound absorbing material 19 in thin-layer form is attached onto the inner wall of the air-outlet duct 18. It should be noted that the plasma display device 100 is mounted to a wall 21 by a wall mounting jig 20 that is coupled onto the back cover 4, when being wall mounted.
However, when the air-outlet duct 18 is provided as indicated in FIG. 6, the flow rate of the air to be exhausted through the fan 10 decreases further because of the resistance in the flow path of the air-outlet duct 18. Further, the gap between the back cover 4 and the wall 21 (wall mounting gap) tends to be narrowed more and more in recent years, and thus it has become difficult to provide the air-outlet duct 18 itself.